1. Field of the Invention
The present invention relates to an absolute position encoder.
2. Description of the Related Art
Incremental encoders and absolute encoders are known as devices for measuring travel distances of objects. The incremental encoders measure relative travel distances and absolute encoders allow for absolute position length-measurement.
In the case of photoelectric encoders, the incremental encoders have incremental tracks with incremental patterns including equally spaced light and dark regions. Based on these patterns, the incremental encoders count light and dark signals to detect relative travel distances. In addition, the incremental encoders may detect absolute travel distances by detecting origin detection patterns provided separately from the above-mentioned incremental patterns with equally spaced light and dark regions, and then detecting relative travel distances from the origin. However, prior to the measurement, a scale must be moved to right and left directions in order to read origin detection patterns.
On the other hand, the absolute encoders have absolute tracks with absolute patterns representing pseudo-random codes such as M-sequence codes and detect absolute positions resulting from reading the absolute patterns for a corresponding object. Unlike the incremental encoders, the absolute encoders does not require any origin detection based on origin detection patterns and may start measurement at a current position when powered on. However, the absolute encoders have a lower detection accuracy than the incremental encoders.
As such, an absolute position encoder is known where an incremental track with equally spaced incremental patterns and an absolute track with absolute patterns representing pseudo-random codes are positioned in parallel on one scale, as disclosed in, e.g., Japanese Patent Laid-Open No. (HEI) 7-286861.
In an encoder of this type, if the pitch of incremental patterns is 40 μm, then the absolute patterns must have a position accuracy of not more than ±20 μm. However, in encoders so configured, it is difficult to form highly accurate patterns since incremental patterns and absolute patterns have a different line width and different density of patterns, respectively.